Melting and boiling points
Methyl chloride, methyl bromide, ethyl chloride and some chlorofluoromethanes are gases at room temperature. Higher members are liquids or solids. Molecules of organic halogen compounds are generally polar. Due to greater polarity as well as higher molecular mass as compared to the parent hydrocarbon, the intermolecular forces of attraction (dipole-dipole and van der Waals) are stronger in the halogen derivatives. That is why the boiling points of chlorides, bromides and iodides are considerably higher than those of the hydrocarbons of comparable molecular mass. The attractions get stronger as the molecules get bigger in size and have more electrons.
Density
Bromo, iodo and polychloro derivatives of hydrocarbons are heavier than water. The density increases with increase in number of carbon atoms, halogen atoms and atomic mass of the halogen atoms.
Which of the following has the highest density?
Answer & explanation
Correct answer: option 4
The correct answer is Option (4) → CCl4
The passage states that the density increases with increase in the number of carbon atoms, halogen atoms and atomic mass of the halogen atoms. All four compounds have one carbon atom and the same halogen (chlorine), so the compound with the most chlorine atoms is the densest.
\( \ce{CH3Cl} \) — 1 chlorine atom.
\( \ce{CH2Cl2} \) — 2 chlorine atoms.
\( \ce{CHCl3} \) — 3 chlorine atoms.
\( \ce{CCl4} \) — 4 chlorine atoms.
So the density rises in the order \( \ce{CH3Cl} \lt \ce{CH2Cl2} \lt \ce{CHCl3} \lt \ce{CCl4} \), and \( \ce{CCl4} \) has the highest density.